A method of the type mentioned above is known, for example, from EP 1 684 989 B1 which is characterized particularly in that the heat-sensitive recording layer is coated on and the protective layer which covers this heat-sensitive recording layer is printed on, both within one machine pass. The cited document is important inasmuch as it combines printing and coating methods for producing a heat-sensitive recording material such as practiced on a coating machine or even on a paper machine. Unfortunately, the cited document does not contain any further suggestions for forming the heat-sensitive recording layer or for forming the protective layer covering this heat-sensitive recording layer.
EP 2 112 000 A1 suggests a heat-sensitive recording material in which a heat-sensitive recording layer and, optionally, a protective layer covering this recording layer is provided on the one side of a substrate and an optional backcoat is provided on the other side of the substrate. According to this known document, a particularly stable, counterfeit-proof print image formed by supplying heat is achieved through the combination of N-(p-toluenesulphonyl)-N′-3-(p-toluenesulphonyloxyphenyl)urea as color acceptor of a leuco dye system on the one hand and, on the other hand, an imino component which forms an anti-fade system in conjunction with the isocyanate component. This document does not address the possible problem of jamming and sticking together due to the effect of moisture.
EP 2 103 444 A1 discloses a heat-sensitive recording material with a particularly soft, high-gloss film which, comprises polypropylene as substrate and on which a heat-sensitive recording layer and a protective layer are applied on one side and a backcoat is applied on the other side. The backcoat is characterized in that it comprises a special acrylic resin or a special copolymer polyester resin, a hydrazine derivative with a hydrazine residual group, and a quaternary ammonium polymer.
Heat-sensitive recording materials of the general type have been known for many years and are steadily gaining in popularity. This may be explained inter alia by the fact that their use as tickets in particular offers great advantages to ticket suppliers. Because the color-forming components in the heat-sensitive recording process reside in the recording material itself, it is possible to employ large numbers of thermal printers which operate without toner or ink cartridges and whose function need no longer be monitored by persons at regular intervals. Accordingly, this innovative technology has had extensive success particularly in public transportation, busses and rail transportation, air travel, stadium and museum ticket kiosks, and parking receipt dispensers. However, it is precisely in this very important area of application of parking receipts that numerous problems persist which have not so far been solved in a convincing manner.
Owing to the fact that parking receipt tickets may be exposed to direct sunlight at very high temperatures when placed behind the windshield, as is frequently required, it happens time and again that the heat-sensitive ticket darkens heavily to the point of turning completely black. The print image produced by the thermal printer is then no longer legible, and the tickets become unusable while still within their period of validity. Poor resistance of the heat-sensitive recording material used for the parking receipt tickets to grease and plasticizers further impairs the legibility of the printed information. However, parking receipt tickets very often come into contact precisely with these substances because grease often adheres to the fingers and hands of the user, and plasticizers are contained in sleeves into which the parking receipt tickets are inserted during their period of use.
Another problem is the risk of jamming of ticket rolls after coming into contact with water. For example, when rolls or fan-folded stacks of parking receipt tickets of heat-sensitive recording material to be dispensed are loaded in automatic ticket dispensers in rainy weather, it may happen that these rolls or fan-folded stacks are dampened by raindrops and this moisture penetrates into the ticket rolls or fan-folded stacks. If the constituents particularly in the outer layers of the parking receipt tickets begin to dissolve, the individual layers within a roll or within the fan-folded stack of parking receipt tickets to be dispensed may stick together resulting in a total breakdown of the automatic ticket dispenser in question. Dust is another problem which arises when heat-sensitive recording material is formed in fan-folded stacks. In particular, heat-sensitive recording materials with highly water-resistant protective layers are often very brittle so that the protective layers can flake off at the cut edges and folded edges. This causes dust and disrupts production.
The problems described above relating to severe background darkening due to direct sunlight, or jamming and sticking together due to the effect of moisture, are even more pronounced in the case of parking tickets which are placed under the windshield wipers of illegally parked cars because they are even more exposed to environmental influences.